Neurotransmitter receptors which mediate their cellular actions through guanine nucleotide binding proteins (G proteins) comprise a large gene superfamily and modulate the activity of a variety of important physiological functions. To better understand the molecular mechanisms of signal transduction mediated by this family of receptors we have utilized gene cloning, permanent gene expression in cultured cells, and site-directed mutagenesis to study the relationship between receptor structure and function and the mechanisms of receptor regulation. Using transfected cell systems expressing homogeneous populations of a single receptor subtype we have demonstrated that G protein-linked receptors are capable of activating multiple independent intracellular signalling pathways and the magnitude of cellular responses to neurotransmitters correlate with the density of receptors present in the cell membrane. These findings have important implications in understanding cellular responsiveness to hormones and neurotransmitters in vivo. Site-directed mutagenesis has allowed for the identification of conserved aspartic acid, cysteine and serine residues in adrenergic and muscarinic acetylcholine receptors which are critical for ligand biding and receptor activation by agonists. These studies have provided evidence that ligand biding to G protein-linked receptor occurs within a pocket formed by the transmembrane helices; that receptors classes which bind amine ligands share common sites for ligand recognition and that agonist activation mechanisms may involve charge transfer across the cell membrane. Finally it is known that beta-- adrenergic receptors are subject to heterologous regulation by hormones such as glucocorticoids and thyroid hormones. Using receptor-specific gene probes we have shown that thyroid hormone regulation of beta1- and beta2-adrenergic receptor density in vivo occurs at the level of gene transcription in a tissue-specific manner. Current work is directed toward elucidating more completely the ligand binding sites of adrenergic and muscarinic acetylcholine receptors and the molecular mechanisms involved in receptor desensitization by agonists.